1. Field of the Invention
The present invention relates generally to implantable prostheses for replacing human skeletal joints, and relates more particularly to a prosthetic component having one or more holes therethrough for receipt of a bone screw or instrument.
2. Background Information
Implantable orthopedic prostheses, in one form, comprise manufactured replacements for the ends and articulating surfaces of the bones of the skeleton. Such prostheses are implanted to repair or reconstruct all or part of an articulating skeletal joint that is functioning abnormally due to disease, trauma, or congenital defect. Among the various articulating skeletal joints of the human body that are eligible to be fitted with implantable orthopedic prostheses, the hip joint and the knee joint are most often treated with such prostheses. The hip and knee joints are major weight bearing joints and degenerate more quickly than other joints in case of abnormality. Also, the hip and knee joints play a critical role in ambulation and quality of life, resulting in great demand for surgical correction of abnormalities.
The human hip joint involves two bones: the femur and the pelvis, each having a smooth articulation surface arranged for articulation against an adjacent articulation surface of the other bone. The femur includes at its proximal extremity a head having a convex, generally spherically contoured articulation surface. The pelvis, in pertinent part, includes an acetabulum having a concave, generally spherically contoured articulation surface. The mutually engaging articulation surfaces of the femur and the pelvis together form, functionally, a ball-and-socket joint.
One or both of the articulation surfaces of the hip joint may fail to act properly, requiring the defective natural articulation surface to be replaced with a prosthetic articulation surface provided by an implantable prosthesis. To fit defects of varying scope, while allowing healthy portions of the hip joint to be conserved, a range of types of orthopedic implants is available. The range extends from total hip prosthesis systems for replacing the articulation surfaces of both the femur and the pelvis, to simpler systems for replacing only the femoral articulation surface. Commonly employed orthopedic hip prostheses include components that fall within one of three principle categories: femoral stems, femoral heads and acetabular cups. A so-called "total" hip prosthesis includes components from each of these categories. The femoral stem replaces the proximal end of the femur and includes a distal stem received within the medullary canal at the proximal end of the femur. The femoral head replaces the natural head and articulating surface of the femur. The acetabular cup replaces the natural socket and articulating surface of the acetabulum of the pelvis. In some designs, the stem and head are an integral, unitary component, but more often the stem and head are separate modular components designed to be assembled to suit the anatomical needs of the patient. A so-called "bipolar hip prosthesis includes only a femoral stem and a head component. The femoral part of the hip joint is replaced with a femoral stem supporting an artificial femoral head. The latter includes an inner head, fixed to the femoral stem, that articulates within an outer head. The outer head articulates directly against the natural acetabulum. Similarly, a so-called "unipolar" hip prosthesis also includes only a femoral stem and a head components. The femoral part of the hip joint is replaced with a femoral stem supporting an artificial femoral head. The femoral head articulates directly against the natural acetabulum while remaining fixed relative to the femoral stem.
The human knee joint involves three bones: the femur, the tibia and the patella, each having smooth articulation surfaces arranged for articulation on an adjacent articulation surface of at least one other bone. The femur includes at its distal extremity an articulation surface having medial and lateral convex condyles separated posteriorly by an intercondylar groove running generally in the anteriorposterior direction, the condyles joining at the distal-anterior face of the femur to form a patellar surface having a shallow vertical groove as an extension of the intercondylar groove. The patella includes on its posterior face an articulation surface having a vertical ridge separating medial and lateral facets, which facets articulate against the patellar surface of the femur and against the medial and lateral condyles during flexion of the knee joint, while the vertical ridge rides within the intercondylar groove to prevent lateral displacement of the patella during flexion. The tibia includes at its proximal end an articulation surface having medial and lateral meniscal condyles that articulate against the medial and lateral condyles, respectively, of the femur. The mutually engaging articulation surfaces of the femur and the patella together form, functionally, the patellofemoral joint, and the mutually engaging articulation surfaces of the femur and tibia together form, functionally, the tibiofemoral joint, which two functional joints together form the anatomical knee joint.
All or part of one or more of the articulation surfaces of the knee joint may fail to act properly, requiring the defective natural articulation surface to be replaced with a prosthetic articulation surface provided by an implantable prosthesis. To fit defects of varying scope, while allowing healthy portions of the knee joint to be conserved, a range of types of orthopedic knee implants is available. The range extends from total knee prosthesis systems for replacing the entire articulation surface of each of the femur, tibia and patella, to simpler systems for replacing only the tibiofemoral joint, or only one side (medial or lateral) of the tibiofemoral joint, or only the patellofemoral joint. Commonly employed orthopedic knee prostheses include components that fall within one of three principle categories: femoral components, tibial components, and patellar components. A so-called "total" knee prosthesis includes components from each of these categories. The femoral component replaces the distal end and condylar articulating surfaces of the femur and may include a proximal stem received within the medullary canal at the distal end of the femur. The tibial component replaces the proximal end and meniscal articulating surfaces of the tibia and may include a distal stem received within the medullary canal at the proximal end of the tibia. The patellar component replaces the posterior side and natural articulating surface of the patella. Sometimes, the patellar component is not used, and the natural articulating surface of the patella is allowed to articulate against the femoral component. A so-called "unicondylar" knee prosthesis replaces only the medial or the lateral femoral condylar articulating surface and the corresponding tibia[ meniscal articulating surface.
The acetabular cup component of a total hip prosthesis is configured to be received and fixed within the acetabulum of a pelvis. The pelvis is prepared to receive the acetabular cup by reaming a concavity in the acetabular bone. The acetabular cup component typically has an outer surface conforming to the concavity reamed in the acetabular bone of the pelvis, and an inner bearing cavity for receiving the head of the femoral component. The head articulates in the beating cavity as a ball-and-socket joint to restore motion to a defective hip joint.
One known type of acetabular cup involves an acetabular shell made of a bio-compatible metal such as titanium or a titanium alloy, and a bearing insert made of a bio-compatible polymer such as ultra-high molecular weight polyethylene. The acetabular shell is shaped generally as a hemispherical cup having a dome, or apex, at a proximal end and an annular rim at a distal end. As used herein, the words proximal and distal are terms of reference that indicate a particular portion of a prosthesis component according to the relative disposition of the portion when the component is implanted. "Proximal" indicates that portion of a component nearest the torso, whereas "distal" indicates that portion of the component farthest from the torso. Between the dome and rim, the acetabular shell comprises a shell wall defined by a generally convex proximal surface and a generally concave distal surface spaced from the proximal surface. The concave distal surface defines a shell cavity having an opening at the rim of the cup for receiving the bearing insert. The bearing insert has a generally convex proximal surface configured to be received and fixed within the acetabular shell in generally congruent engagement with the concave distal surface of the shell wall. The bearing insert also has a bearing cavity that opens distally for receiving the head of the femoral component. The bearing cavity is defined by a generally spherical concave bearing surface having a radius similar to that of the femoral head component. The concave bearing surface articulates against the surface of the spherical femoral head component.
Acetabular shells of the type described can be affixed to the acetabular bone by bone screws or bone cement. If bone screws are elected, the screws are driven into the bone through the screw holes before the bearing insert is placed into the shell. The shell also can be affixed by a combination of bone screws and bone cement. The acetabular shell can be provided with more screw holes than typically would be used by the implanting physician. This provides a selection of sites for placement of the bone screws, as may be dictated by the condition of the patient's pelvic bone or by the physician's preference. Some provided screw holes may receive a screw while others do not. For reasons explained below, it is desirable to provide means for occluding those screw holes that will not receive a screw.
Commonly, acetabular shells of the type described also include a dome hole at the apex. A typical dome hole is coaxially aligned with the axis of symmetry of the acetabular shell and extends through the shell wall from the concave distal surface to the convex proximal surface of the acetabular shell. Often, the dome hole is internally threaded or otherwise configured for receiving an instrument for holding and positioning the acetabular shell during implantation. Also, many physicians use the dome hole to obtain visual or tactile access to the reamed acetabular bone during implantation of the acetabular shell. Such access allows the physician to confirm that the acetabular shell is fully seated in engagement with the reamed bony surface of the acetabulum. As with the screw holes, for reasons explained below, it is also desirable to provide means for occluding the dome hole.
The bearing insert is usually designed to be received within the acetabular shell in nonarticulating relative relationship. Nevertheless, a small amount of unintended relative motion is believed to occur between the bearing insert and the acetabular shell in response to the varying load borne by the acetabular cup during use. Such small relative motion, or micro-motion, may result in wear at the interface between the bearing insert and acetabular shell that generates fine polyethylene or metal debris. According to some hypotheses, such debris can migrate out of the acetabular cup and contact bone, possibly resulting in osteolysis, which ultimately can lead to bone resorption and possible loosening of the acetabular prosthesis. One apparent pathway for the migration of debris out of the acetabular shell is through open screw holes. Another apparent pathway is through an open dome hole.
The tibial component of a total knee prosthesis is configured to be received upon and fixed to the proximal end of the tibia. The tibia is prepared to receive the tibial component by resecting part of the proximal end of the tibia to leave a substantially horizontal planar bony plateau. Sometimes the exposed medullary canal at the proximal end of the tibia is also reamed to receive a stem portion of the tibial component. The tibial component typically includes a plate portion having an inferior planar surface conforming to the resected bony plateau at the proximal end of the femur. The plate portion may or may not include a depending stem or keel for receipt withing a prepared tibial medullary canal. Commonly, a meniscal bearing insert is received atop the plate portion of the tibial component to provide an artificial meniscal articulating surface for receiving the condylar surfaces of the femoral component of the total knee prosthesis. The femoral condylar articulating surfaces articulate against the tibial meniscal articulating surface to restore motion to a defective knee joint.
One known type of tibial component involves a tibial plate made of a biocompatible metal such as titanium or a titanium alloy, and a meniscal bearing insert made of a bio-compatible polymer such as ultra-high molecular weight polyethylene. The tibial plate is shaped generally as a flat plate having a perimeter that generally conforms to the transverse sectional perimeter of the resected proximal tibia. The tibial plate includes a planar distal, or inferior, surface for engaging the resected proximal tibia, and a proximal, or superior, surface for engaging and receiving the meniscal bearing insert. One or more screw holes may extend through the plate portion from the superior to the inferior surface. The bearing insert has an inferior surface that engages the superior surface of the plate portion, and may include locking tabs or other means for fixing the bearing insert to the plate portion against relative movement.
Tibial plates of the type described can be affixed to the resected tibial bone by bone screws or bone cement. If bone screws are elected, the screws are driven into the bone through the screw holes before the bearing insert is placed atop the plate portion. The plate also can be affixed by a combination of bone screws and bone cement. Sometimes the plate can be provided with more screw holes than typically would be used by the implanting physician. This provides a selection of sites for placement of the bone screws, as may be dictated by the condition of the patient's tibial bone or by the physician's preference. Some provided screw holes may receive a screw while others do not. For reasons similar to those discussed above regarding acetabular shells, and again briefly below, it is desirable to provide means for occluding those screw holes that will not receive a screw.
The tibial bearing insert usually is designed to be received atop the tibial plate in nonarticulating relative relationship. Nevertheless, a small amount of unintended relative motion is believed to occur between the bearing insert and the tibial plate in response to the varying load borne by the tibial component during use. Such small relative motion, or micro-motion, may result in wear at the interface between the bearing insert and acetabular shell that generates fine polyethylene or metal debris, similarly to the hypothesized phenomenon discussed above regarding acetabular shells. One apparent pathway for the migration of debris from the superior surface of the tibial plate is through open screw holes. In some total knee prostheses, the bearing insert is intended to articulate on the tibial plate in sliding or rotating relationship. Such knee prostheses are known as "mobile bearing" knees. The possibility of wear debris being generated in such knee prostheses is apparent.